Cystosarcoma phyllodes (CP) of the breast is a rare biphasic tumor composed of benign epithelium and a spindle cell stroma. Biologic behavior of CP cannot be predicted with certainty on the basis of morphologic criteria only. We studied immunohistochemical expression of basic fibroblastic growth factor (bFGF), urokinase, Ki67, p53 protein, and microvessel density in stromal and epithelial components of 14 low-grade CP (LCP) and 9 high-grade CP (HCP). bFGF was more often positive in LCP than in HCP. The stroma was positive for bFGF in 86% of LCP and 67% of HCP, and the epithelium was positive in 64% of LCP and 14% of HCP. Urokinase was positive in stromal cells of 86% of LCP and 93% of HCP. The epithelial positivity for urokinase in both groups resembled closely that of the stroma. p53 protein was more often positive in stromal cells of HCP (67%) than in LCP (50%). Ki67 was positive in the stroma of 43% of LCP and 89% of HCP and in the epithelium of 14% of LCP and 33% of HCP. There was no significant difference in microvessel density (MVD) in low- and high-grade lesions. Our study demonstrates that stromal Ki67 and p53 immunohistochemical positivity are more often associated with high-grade tumors. The positive immunostaining for bFGF, urokinase, Ki67, and p53 in stroma and epithelium of the majority of CP supports the existence of epithelial-stromal interactions and recognizes epithelium as an integral part of this tumor.